In printing industries, an off-set printing method is widely carried out which employs a planographic printing plate obtainable from a light sensitive planographic printing plate material. In the off-set printing method, printing is carried out employing a planographic printing plate which is obtained by imagewise exposing a light sensitive planographic printing plate material, and developing the exposed printing plate material to form, on the planographic printing plate, image portions which receive printing ink and non-image portions which receive dampening water and repel printing ink. The resulting planographic printing plate is plate processed with a plate protecting solution before or during printing in order to protect the surface of the printing plate.
Thus, the plate processing may maintain hydrophilicity of the non-image portions for a period from development until printing, to prevent stain or scratch produced due to fingerprint, oil and fat or dust which may be deposited on the plate surface while correction such as rewriting or erasing is carried out or the printing plate is handled, for example, it is mounted on a plate cylinder of a printing press, and to prevent stain due to oxidation of the non-image portions after relatively long term storage.
Hitherto, an aqueous solution of gum arabic, cellulose gum, or a water-soluble resin having a carboxyl group in the molecule has been used as the plate protecting solution.
In addition to the above, a plate protecting solution containing a phosphoric acid-modified starch is disclosed in Japanese Patent O.P.I. Publication No. 62-11693, and a plate protecting solution containing a carboxyalkylated starch is disclosed in Japanese Patent O.P.I. Publication Nos. 62-7595. Further, a plate protecting solution containing phosphoric acid or its derivatives (including phosphonic acid) are known (see Japanese Patent O.P.I. Publication No. 62-11693.).
A plate protecting solution containing a polyoxyethylene-containing surfactant is proposed (see Japanese Patent O.P.I. Publication No. 11-265074), a plate protecting solution containing a hydrocarbon with a specific chemical structure is proposed (see Japanese Patent O.P.I. Publication No. 11-288106), a plate protecting solution containing a soybean polysaccharide and an aldehyde compound is proposed (see Japanese Patent O.P.I. Publication No. 2003-182256), and further, a plate protecting solution is known in which a phosphonic compound is added as a chelating agent to these plate protecting solutions.
As a support of a light sensitive planographic printing plate material, one having high hydrophilicity, high water retention property, and excellent adhesion to the light sensitive layer is desired, and an aluminum plate whose surface is subjected to roughening treatment is generally used. For a developer for a printing plate material with an aluminum support, a developer containing silicate has been used in view of pH buffering effect or hydrophilization effect, however, sludge, which causes stain on the developed printing plate material, may be produced in the developer when a large number of printing plate materials are developed. A developer, which does not substantially contain silicate, is known (see Japanese Patent O.P.I. Publication No. 2002-351094), but it is not satisfactory in minimizing stain occurrence.
As a light sensitive layer, a light sensitive containing a polymerizable ethylenically unsaturated compound is known. A photopolymerization type light sensitive planographic printing plate material, in which such a light sensitive layer is provided on the aluminum support, is advantageously used in view of high sensitivity or printing durability.
The photopolymerization type light sensitive planographic printing plate material provides high sensitivity as described above. However, when a large number of photopolymerization type light sensitive planographic printing plate materials are developed employing a conventional automatic developing machine to prepare a large number of printing plates, and printing is carried out employing the resulting printing plate, suspended, and resumed, stain is likely to occur at non-image portions of the printing plate. Even a printing plate, which has been plate processed with the plate protecting solutions as described above, is insufficient in preventing such stain occurrence.
In recent years, environmental protection has been sought also in the printing industries, and printing ink containing no petroleum type volatile organic compound (VOC) has been developed and used. When such a printing ink is employed, it is especially difficult to prevent stain occurrence as described above.